Digital signatures can be used to verify that an electronic document has not been altered since the creation of the digital signature based on the electronic document. In many systems, digital signature creation includes two parts, hashing and encryption of the produced hash. Hashing involves computing a hash (message digest) function over a sequence of encoded binary data.
An electronic document contains information that can be encoded as one or more different encodings. An encoding is binary data representing the digitally encoded form of the document information. For example, a document may be encoded as one of many PDF (Portable Document File developed by Adobe Systems of San Jose, Calif.) encodings or one of many XDP (XML, extensible markup language, Data Package) encodings.
The encoding is used to generate a hash using any hashing function. Once a hash has been computed, the hash is encrypted using an encryption algorithm. The recipient of the document can verify the digital signature by decrypting the digital signature to obtain the hash and comparing the hash to a new hash generated from the document using the same hashing function as the digital signature author. If the hashes match, the document is verified as being not altered from when the digital signature was produced.
Conceptually, a digital signature should apply to the information present in the document and not to the specific document encoding. Since current digital signature algorithms use binary data of a specific encoding to generate the digital signature, an already digitally signed document cannot be converted into a different encoding without invalidating the signature. This signature conversion problem arises when the desired encoding is a different encoding from the original document encoding, i.e. different PDF encoding conversion, different XDP encoding conversion, or PDF to XDP conversion. Even reordering XML data in a XDP file invalidates the signature. The signature conversion problem has been traditionally solved by using a canonical encoding (data is ordered in a predetermined and repeatable order) to generate a digital signature. The signature validation now requires an extra step of converting the document to the canonical encoding before reproducing the hash for hash comparison in validating the signature. This canonical conversion step can be computationally expensive. There exists a need to more efficiently validate digital signatures for multiple document encodings.